Projection displays offer an alternative to flat-screen displays such as plasma and TFT displays, in which the image is directly generated on the display surface. In one type of image projection system, a video signal is processed to create an image in a small display panel, and this image is magnified and projected onto a projection display, or screen. In a scanning projection system, on the other hand, the image is scanned in a pixel-wise or line-wise manner onto the projection display. A screen can be a backdrop in the case of a front-projection system—the projector or ‘beamer’ is in front of the screen and is usually positioned behind the viewer, for example suspended from the ceiling, in order to project the image onto the front of the screen; or a television screen in the case of a rear-projection system, in which all components are contained in a single device, and the image is projected onto the screen from behind.
State of the art projection displays often utilise a high-intensity discharge (HID) lamp or ultra-high pressure (UHP) lamp to produce the required bright beam of white light. The white light is then either split into the light primaries red, blue and green by means of dichroic filters, or passed through a colour wheel with red, blue and green colour filters, and the primaries are then directed at a two-dimensional display panel to generate a sequence of ‘sub-images’ in the primary colours. The display panel can be, for example, an array of micro-mirrors as in DLP® (Digital Light Processing®) or an LCD or LCoS (Liquid Crystal on Silicon) array. The display panel can also be referred to as a ‘micro-display’, since an image is first rendered on this very small element before being projected onto a screen or backdrop for viewing by a user. For example, in a DLP system, the light primaries are directed in quick succession at the display panel to create red, blue and green sub-images which are projected onto the display and perceived as a combined image by the viewer.
The state of the art projection systems that use UHP lamps have the disadvantage of requiring a very complex optics system for obtaining the primaries from the white light source. Another drawback shared by these systems is that the lamp may fail if not driven according to certain criteria. Furthermore, such a lamp can be subject to blackening as the lamp gets older, so that the projected image eventually suffers from a decline in quality. Replacement lamps are also quite expensive from the point of view of the consumer.
Instead of using a single white UHP light source, it is possible to use three primary colour light sources such as red, green and blue lasers. Up until recently, the lasers available for use in laser-based projection systems have been relatively large and expensive, particularly those for generating green and blue light, since green or blue laser light must be generated using an additional frequency conversion. However, affordable compact semiconductor diode lasers that use frequency doubling to obtain blue or green light from an infrared laser are coming onto the market. An example of such a semiconductor laser source is the electrically pumped Vertical External Cavity Surface-Emitting Laser, or VECSEL. Directly-emitting blue lasers are already available for applications such as BluRay® and HD DVD (High-Definition Digital Versatile Disk), as will be known to a person skilled in the art. Compact projection units are being developed for such small lasers. An example of such a projection unit uses a micro scanning mirror that oscillates to deflect a modulated laser beam horizontally and vertically in order to generate an image on a backdrop.
Obvious advantages of laser-based projection systems are their compact size, energy efficiency, and low maintenance. Particularly the smaller types of laser projection system, such as those intended for portable/mobile use, do not require any complicated filtering or focussing optics.
The light output of a laser such as those described above can be directly controlled by application of a modulation signal, for example by modulating the input current to the laser as required. Since a high internal conversion efficiency is only obtained at high energy density, these types of laser devices are operated in a pulsed mode, in which short pulses are applied to the laser, resulting in a sequence of short light pulses being generated. As will be known to a person skilled in the art, the output energy is thereby concentrated, giving an increased energy density at a certain average level.
To obtain an image in which the pulsed method of driving is not noticeable to the viewer, the lasers must be pulsed at a sufficiently fast rate corresponding to the pixel frequency. This leads to bandwidth problems when such lasers are to be used to project images with high resolution and low flicker, for example movies encoded using PAL or HD standards. PAL encodes 720 by 576 pixels for every image, while high-definition television can encode at 1280 by 720 pixels or 1920 by 1080 pixels. The projected image must be refreshed at a rate of about 90 Hz in order to avoid flicker. For a PAL image, this results in a modulation bandwidth of at least 20 MHz. Projection of full HD images can require a bandwidth of up to 93 MHz. Obviously, such high bandwidth requirements result in expensive electronics overhead and control software.
It is therefore an object of the invention to provide a cost-effective, straightforward and efficient method of image projection.